This invention relates generally to a system and method for transmitting power to the wheels of a motor vehicle.
Vehicle drive systems in the prior art employ an on/off braking device, which changes the transmission ratio between front driven axles and rear driven axles. Other systems use a mismatch in final drive ratios and controllable braking devices, i.e. two or more brakes, to vary the magnitude of torque transmitted to the driven axles.
In the prior art, the speed and torque transmitted to the left and right rear wheels are controlled by a differential mechanism having two slipping brakes (electromagnetic or hydraulic), one for the right axle shaft, the other brake for the left axle shaft.
U.S. Pat. No. 4,981,190 discloses a four wheel drive vehicle that directs torque to desired wheels by using a brake system. The brake system includes a brake control unit for controlling disk brakes. If one of the wheels slips, the brake control unit stops the slip and sends torque to other wheels.
U.S. Pat. No. 6,712,730 discloses a torque coupling that connects the secondary driving wheels of an automotive vehicle indirectly to the power unit of the vehicle through a variable torque coupling, while the primary driving wheels on the vehicle are connected directly to the power unit. The coupling compensates for variations in the angular velocity of the primary and secondary wheels while still transferring torque to the secondary wheels. The coupling includes a magnetic particle brake organized about an axis and a planetary gear set organized about the same axis. The coupling has two paths through which the torque is transferred, one a mechanical path and the other a brake path. Most of the torque is transferred through the mechanical path, while the brake path accommodates for slippage and controls the proportion of torque delivered to the secondary wheels, with the control being solely dependent on the magnitude of the current directed through the brake.
It would be desirable to provide a system and method for varying the magnitude of torque transmitted to the input of a differential mechanism and adjusting the magnitude of differential torque applied to the left and right rear wheels by controlling wheel brake torque at each of the rear wheels in accordance with current vehicle handling requirements.